Enzymes have received considerable attention because of their ability to catalyze various reactions which occur in living organisms. The enzymes may also be isolated and used in industrial applications. For example, the enzyme glucose isomerase is extensively used to convert glucose to fructose in the manufacture of high fructose corn syrup.
Because enzymes are water soluble and generally unstable, they are difficult to remove from the reaction mixture for reuse. Accordingly, this increases the cost of using enzymes on a commercial scale. A solution to this problem has been to immobilize the enzyme by using a porous oxide support on which the enzyme is deposited.
In certain applications it is desirable to use (and immobilize) whole cells of micro-organism. The use of whole cells has the advantage that the cell acts as a carrier for the enzyme and one does not have to extract the enzyme from the cell. If whole cells are to be immobilized on a porous oxide support, the support must have pores which are large enough to accommodate the whole cells. If the pores are too small, the microbial cells or enzymes will plug the pores. Additionally, small pores may cause the fluid with which the whole cells are to be contacted to "hold-up" in the pores, thereby reducing the production rate of product because fresh fluid cannot reach the enzyme present in the whole cell.
Conventional silica, alumina or silica-alumina supports are microporous with micropores of about 0.001 to about 0.03 microns in diameter. These pores are too small to accommodate microbial cells since typically microbial cells are larger than 1 micron. The prior art indicates that pore diameters of about 1 to about 25 microns are needed to adequately accommodate microbial cells. Thus, U.S. Pat. No. 4,581,338 discloses a process for preparing an enzyme support with large macropores. The process involves making a mixture of diatomite, a solvent, a fluxing agent and an organic burnout material, forming the mixture into generally spherical balls and then calcining the balls at a temperature of about 700.degree. to 2300.degree. F. Diatomite or diatomaceous earths are interchangeable terms which refer to sedimentary materials composed of the skeletal remains of single celled aquatic plants called diatoms. The resultant support is stated to have an average pore diameter of at least 8 microns.
Applicant has taken a different approach to making a macroporous support by forming a mixture of an aluminum sol and a diatomaceous earth, forming spheres from this mixture by dropwise dropping the mixture into an oil tower, followed by calcination of the spheres at a temperature of about 1,000.degree. to about 1450.degree. C. The resultant support has pores which are at least 5 microns and which are interconnected. It is important that the pores be interconnected; otherwise whole cells cannot enter the pores and the fluid which is to contact the cells will not be able to diffuse into the large pores. Accordingly, the use of applicant's process produces interconnected macropores without using a burnout material. Applicant's process also produces a uniform, spherical product as compared to the prior art's nearly spherical product.
The prior art also reveals that alumina spheres may be prepared using a suspension of an alumina powder. For example, U.S. Pat. No. 4,514,511 teaches that a mixture of an alumina powder and an aluminum salt can be used to form spheres with pores having a diameter from about 0.2 to 15 microns. However, these pores are closed which would make them useless as a support for whole cells. Applicants are, therefore, the first to prepare an alumina/diatomaceous earth macroporous support, having interconnected pores.